'vicesimal' would be more correctly derived from Latin 'uicesima (pars)' =
1/20. But, tho my dictionary does give 'vicesimal' as an alternative to
'vigesimal', I fear the latter is too entrenched in the language now.

'heccaedecadic' would be /hEksIdI'k&dIk/ and spellt/ spelled 'heccedecadic'
in north America, I assume :)